An organic EL is a spontaneous light emitting device which utilizes the principle that a fluorescent substance emits light by energy of recombination of holes injected from an anode and electrons injected from a cathode when an electric field is applied.
Since an organic EL device of the laminate type driven under a low electric voltage was reported by C. W. Tang of Eastman Kodak Company (C. W. Tang and S. A. Vanslyke, Applied Physics Letters, Volume 51, Pages 913, 1987), many studies have been conducted on organic EL devices using organic materials as the constituting materials.
Tang et al used a laminate structure using tris(8-hydroxyquinolinato)aluminum for the light emitting layer and a triphenyldiamine derivative for the hole transporting layer. Advantages of the laminate structure are that the efficiency of hole injection into the light emitting layer can be increased, that the efficiency of forming excited particles which are formed by blocking and recombining electrons injected from the cathode can be increased and that excited particles formed within the light emitting layer can be enclosed. As the structure of the organic EL device, a two-layered structure having a hole transporting (injecting) layer and an electron transporting and light emitting layer and a three-layered structure having a hole transporting (injecting) layer, a light emitting layer and an electron transporting (injecting) layer are well known. To increase the efficiency of recombination of injected holes and electrons in the devices of the laminate type, the structure of the device and the process for forming the device have been studied.
As the light emitting material, chelate complexes such as tris(8-quinolinato) aluminum, coumarine derivatives, tetraphenyl- butadiene derivatives, bisstyrylarylene derivatives and oxadiazole derivatives are known. It is reported that light in the visible region ranging from blue light to red light can be obtained by using these light emitting materials and development of a device exhibiting color images is expected (For example, Japanese Patent Application Laid-Open Nos. Heisei 8(1996)-239655, Heisei 7(1995)-138561 and Heisei 3(1991)-200889.
Devices using bisanthracene derivatives as the hole transporting material or the light emitting material are disclosed in the U.S. Pat. No. 3,008,897 and Japanese Patent Application Laid-Open No. Heisei 8(1996)-12600. Although bisanthracene derivatives can be used as the material emitting blue light, the efficiency of light emission and the life are insufficient for practical applications. In Japanese Patent Application Laid-Open No. 2001-207167, a device using an aminoanthracene derivative as the material emitting green light is disclosed. However, the organic EL device using this material cannot be used for the practical applications since the device has poor heat resistance due to the low glass transition temperature of the material and light emission of a long life and a high efficiency cannot be obtained. Other organic devices of a long life and excellent properties are recently reported. However, the life and the properties are not always sufficient. Therefore, the development of an organic EL device exhibiting more excellent properties have been strongly desired.